1. Field of the Invention
The invention relates to a device for mixing gas and/or vapor streams containing disperse liquid components in a column-type fixed bed reactor.
2. Discussion of the Background
During the catalytic treatment of hydrocarbon products, the sulfur-, nitrogen-, and oxygen-containing compounds in the gas/vapor/liquid mixture being fed are decomposed by catalytic treatment with hydrogen-containing gases. At the same time, cracking reactions take place, and unsaturated compounds are hydrogenated. Overall, the reactions are exothermic, so that the reaction mixture must be cooled between the individual catalyst beds in the reactor, by mixing-in cold gas or quench gas.
Stringent requirements are placed on the apparatus for mixing the reaction product with quench gas after the product leaves a catalyst bed and for redistributing the cooled mixture for feeding to the next lower bed. These requirements relate to the mixing effect; cooling effect; redistributing effect; pressure drop or loss (to be kept minimal); structure volume (to be minimized); and insensitivity to contamination and fouling.
The reasons for these requirements are as follows. The parameters of the quench gas feed are chosen such that even the hottest stream within the mixture flow will be adequately cooled before entering the next bed, whereby inadmissibly high temperatures will not occur in the subsequent conversion. Accordingly, any nonuniform mixing and/or cooling and/or redistribution will increase the amount of quench gas required. Further, parts of the reaction mixture will be overcooled, resulting in a reduced reaction rate for them, and increasing the catalyst volume required to achieve the desired overall conversion. Thus, nonuniform mixing, cooling, and/or redistribution results in higher capital costs (larger reactor and catalyst volumes) as well as higher operating costs (additional quench gas required).
The mixing, cooling, and redistribution effects of a device can generally be improved by increasing the energy expended therefor with the consequence of an increased pressure drop in the mixture stream or by lengthening the mixing and cooling sections with the consequence of greater apparatus volume. Increased pressure drop increases the operating costs, and greater apparatus volume increases the capital costs.
Finally, the reactor system should not be subject to problems arising from contamination in the form of catalyst dust, coke, etc. Every time production must be stopped to clean reactor elements (which are generally not readily accessible), there is a substantial decrease in reactor availability and thus a decrease in profitability.
An apparatus has been proposed for use in fixed bed reactors for mixing and redistributing fluids. See, for example, "Die Katalytische Druckhydrierung von Kohlen, Teeren and Mineraloelen" (Catalytic pressure hydrogenation of coals, tars, and mineral oils). Springer-Verlag, 1950, p. 241. According to this reference, the cold gas is introduced from the top cover through a vertical pip flows from an annular space on the wall of the reactor vessel into the hot gas, and mixes with the hot gas as it travels to the center of the furnace. The flow then goes through a return path, with passage through so-called "cold gas baffles", and is passed on to the next catalyst layer as a mixture with uniform temperature, the passage being through a perforated plate.
In German Pat. No. 1,965,435, a device for mixing two streams before their entry into a bed of solids is described, wherein a mixing chamber with flow guide means and additional distribution plates (or column trays) is provided between individual catalyst layers. In this way, quench gas and product streams are fed to the so-called "quench box" through at least two entrance openings. Intimate mixing and temperature equalization are accomplished in one or more distribution structures or devices, e.g., in a standard spout tray.
Both of these proposals have the serious drawback that there are macroscopically large nonuniformities in the incident product which has been pre-mixed with quench gas, in particular there are so-called "strands" of flow, occurring particularly as a result of partial, local de-mixing of gas, vapor, and liquid in the preceding catalyst bed, and these nonuniformities are not removed by intensive mixing in the individual partial streams, even with very large expense.